1. Technical Field
The present disclosure pertains to a transconductor circuit having increased linearity and reduced noise.
2. Description of the Related Art
One approach that has been used for providing a transconductor circuit, applied for example in a communication system, such as in highly integrated tuners for radio frequency reception, is shown in the circuit diagram of FIG. 1. The transconductor circuit as shown in FIG. 1 may be used as a broadband low noise transconductor suitable for high source impedance (e.g., a tuned high-Q RF filter) and typically includes a MOS transistor differential pair. Particularly, the circuit according to FIG. 1 includes a first transistor T1 and second transistor T2 each of the MOS type that are coupled to form a differential pair or differential amplifier. The transistor T1 includes a control node at its gate that is coupled to a first input node INp, and the transistor T2 includes a control node at its gate which is coupled to a second input node INn. The controlled paths of the transistors T1 and T2 are coupled with each other at the source nodes of the transistors which are both coupled to a current source S. At the drain nodes of the transistors T1 and T2 respective output signals may be provided, for example supplied to a mixer circuit.
With the circuit of FIG. 1, input voltage signals at the input nodes INp and INn may be transferred in current output signals provided at the controlled paths of the transistors T1 and T2. However, there are often needs for providing a steep input characteristic of the transconductor circuit while providing at the same time low output current noise. Moreover, the characteristic should be mostly linear that is, however, in contrast to providing low output current noise. A differential pair consisting of transistors of the bipolar type is not preferred due to its base current noise.
Another approach that has been used for providing a transconductor circuit is shown in FIG. 2. According to the transconductor circuit of FIG. 2, the circuit includes a transistor T1a having a control node that is coupled to an input node INp for receiving a first input signal. The second transistor T1b has a control node that is coupled to the second input node INn for receiving a second input signal of the transconductor circuit. Both transistors T1a and T1b are of the MOS type. The controlled paths of the transistors T1a and T1b are coupled to a respective current source S1 and S3. A second pair of transistors T2a and T2b are of the bipolar type and have respective control nodes which are coupled to the controlled path of transistor T1a and the controlled path of the transistor T1b, respectively. The controlled paths of the transistors T2a and T2b are each coupled to a respective resistor R and are coupled with each other through the resistors R to form a differential amplifier or differential pair. The controlled paths of both transistors T2a and T2b are connected to a current source S2. At the collector nodes of each transistor T2a and T2b a respective output current is provided, which may be supplied to a mixer of a tuning circuit. In such application, an antenna voltage signal may be provided at one of the input nodes INp and INn.
If a transconductor circuit is used to drive a so-called switching quartet of a Gilbert mixer, the circuit scheme of FIG. 2 is typically preferred in BiCMOS processes, because the lower capacitances at the collector of the NPN transistor used for transistors T2a and T2b compared to the drain capacitances of the NMOS transistor used for transistors T1 and T2 (FIG. 1) leads to lower radiation of the local oscillator clock via the input nodes and faster switching of a subsequent mixing quartet